1. Field of the Invention
Embodiments of the present invention relate to apparatus, systems and methods for mixing of dry compositions including hydratable agents and optionally gelling agents with base fluids to form hydrated viscous fluids or hydrated gelled viscous fluids, where the hydrated fluids have application in fracturing and similar applications.
more particularly, embodiments of the present invention relate to apparatus, systems and methods for mixing of dry compositions including hydratable agents and optionally gelling agents with base fluids to form hydrated viscous fluids or hydrated gelled viscous fluids, where the hydrated fluids have application in fracturing and similar applications, where the hydrated fluids are formed using a vortex mixing apparatus capable of efficient and effective on-site and on-demand mixing and hydrating the dry compositions into the base fluid without unwanted clumping.
2. Description of the Related Art
Many treatments and procedures are carried out in the oil industry utilizing high viscosity fluids to accomplish a number of purposes. For example, in the oil industry, high viscosity aqueous well treating fluids or gels are utilized in treatments to increase the recovery of hydrocarbons from subterranean formations, such as by creating fractures in the formation. High viscosity aqueous fluids are also commonly utilized in well completion procedures. For example, during the completion of a well, a high viscosity aqueous completion fluid having a high density is introduced into the well to maintain hydrostatic pressure on the formation which is higher than the pressure exerted by the fluids contained in the formation, thereby preventing the formation fluids from flowing into the wellbore. High viscosity treating fluids, such as fracturing gels, are normally made using dry gel additives or agents which are mixed with water or other aqueous fluids at the job site. Such mixing procedures have some inherent problems, particularly on remote sites or when large volumes are required. For example, special equipment for mixing the dry additives with water is required, and problems such as chemical dusting, uneven mixing, and lumping result. The lumping of gels occurs because the initial contact of the gel with water results in a very rapid hydration of the outer layer of particles which creates a sticky, rubbery exterior layer that prevents the interior particles from contacting water. The net effect is formation of what are referred to as “gel balls” or “fish eyes”. These hamper efficiency by lowering the viscosity achieved per pound of gelling agent and also by creating insoluble particles that can restrict flow both into the well formation and back out of it. Thus, simply mixing the untreated gel directly with water is not a very successful method of preparing a smooth homogeneous gel free from lumps.
A method directed to solving this problem is to control particle size and provide surface treatment modifications to the gel. It is desired to delay hydration long enough for the individual gel particles to disperse and become surrounded by water so that no dry particles are trapped inside a gelled coating. This can be achieved by coating the gel with materials such as borate salts, glyoxal, non-lumping HEC, sulfosuccinate, metallic soaps, surfactants, or other materials of opposite surface charge to the gel. A stabilized gel slurry (SPS), also referred to as a liquid gel concentrate (LGC), is the most common way to improve the efficiency of a gel addition to water and derive the maximum yield from the gel. The liquid gel concentrate is premixed and then later added to the water. U.S. Pat. No. 4,336,145 disclosed a liquid gel concentrate comprising water, the gel, and an inhibitor having the property of reversibly reacting with the hydratable gel in a manner wherein the rate of hydration of the gel is retarded.
U.S. Pat. No. 4,722,646 disclosed a hydrocarbon carrier fluid, rather than water, so higher quantities of solids can be suspended. The hydrocarbon-based liquid gel concentrate is later mixed with water in a manner similar to that for aqueous-based liquid gel concentrates to yield a viscosified fluid, but hydrocarbon-based concentrates have the advantage of holding more gel.
U.S. Pat. No. 4,828,034 disclosed one method and system for on-demand mixing of a fracturing gel, in which a fracturing fluid slurry concentrate is mixed through a static mixer device on a real-time basis to produce a fully hydrated fracturing fluid during the fracturing operation. This process utilizes a hydrophobic solvent, which is characterized by a hydrocarbon such as diesel as in the hydrocarbon-based liquid gel concentrates described above.
U.S. Pat. No. 5,190,374 disclosed method and apparatus for substantially continuously producing a fracturing gel, without the use of hydrocarbons or suspension agents, by feeding the dry polymer into an axial flow mixer which uses a high mixing energy to wet the polymer during its initial contact with water.
U.S. Pat. No. 5,382,411 disclosed apparatus and method for continuously hydrating a particulated polymer and producing a well treatment gel.
Inverted emulsions of water soluble polymers are currently used in subterranean treatments such as hydraulic fracturing operations. Similarly polymer gelling agents are prepared using oil based slurries. The concentrates contain hydrocarbons and surfactants. Elimination of the hydrocarbon and the surfactants is desired for cost (material and shipping) and for environmental reasons. The mixers currently used are very costly and are often part of complicated systems.
Dry polymer mixing units have been patented and introduced into field operations. U.S. Pat. Nos. 5,190,374 and 7,104,328 disclose two different mixers. SNF Floerger offers a Polymer Slicing Unit (PSU) described in detail at the website (http://www.snf-oil.com/Equipment.html). The mixer described in U.S. Pat. No. 5,190,374 is reported to fail due to polymer build-up inside the mixer. The mixer described in U.S. Pat. No. 7,104,328 and the PSU both provide shear by a rotating impeller. The mixer described in U.S. Pat. No. 7,104,328 is used to provide a substantially hydrated polymer solution for a continuous mix on demand fracturing operation. The PSU is described to deliver up to a 1.5% polymer solution. Both mixers are used in conjunction with holding tanks to allow additional time for more complete polymer hydration.
While numerous methods and systems have been put in practice for downhole fracturing of oil and gas wells, there is still in need in the art for a less expensive and simpler methods and systems for on-demand hydrating dry hydratable powders on site.